Information in twisted light New Scientist has a story on the use of twisted light to store multiple bits of information per photon. What is this technology, and what are the information density limits per photon?
"MOZART and Schrödinger flew through the air over Vienna recently. Their digital images were encoded in twisted green light, marking an important step towards long-distance communication in free space. Light offers the best way to communicate between Earth and orbiting satellites, but atmospheric turbulence can destroy the signal. Polarised light is resistant to the effects of turbulence, but polarised photons can carry only one bit of information apiece. So researchers have looked for other properties of light that could boost the bit rate. One solution is twisted light, in which the wavefront of light spirals around a central axis as it travels. There is no limit to the number of twists for each photon, so they can theoretically store boundless amounts of information."
 A: One way to conceptualize "twisted light" is to think of a ball being thrown in the air. Usually you spin the ball slightly as you throw it, well imagine spinning the ball a specific amount to encode some information. Then you could build a ball cannon that sets the amount of spin for each ball it fires, that encodes information on each ball. For a binary system, you could say "no spin" = 0 and "spinning" = 1. You could scale this to any fine-grained amount of spin you want. The cool part (like the actual study) is that even if wind blows the ball around, the spin is still there when you catch it: the information survives.
Now the real version, the balls are photons and they can carry angular momentum. The amount of angular momentum each photon carries, can be used to encode information. True, there is no limit to the amount of angular momentum for a given photon, but each photon can only have one value so it can only carry one value. How much actual data that corresponds to depends on your encoding scheme and data compression etc.
